Hi. I am working on a steel structure and i have some issues with the seismic loads.
That are the flexure MZ diagrams for the pure seismic load case in X direction (longitudinal).
It caught my attention the results for the beam element marked in green.
The masses were defined as follows, for X,Y,Z:
If i delete the masses in Y i don´t observe this problem anymore.
And i don´t understan if this is correct, because the response spectrum is defined for X direction:
In anycase, i don´t undestrand either why i have this problem in the green beam and not in the purple beam, considering that the loads are exactly the same.
Also, the resultant base shear shows significant differences, depending if the Y masses are considered or not.
I appreciate any help.
Thanks!
Attach that model please.
Cruce Av A_Rev B1_EO_con_masa_Y.stdCruce Av A_Rev B1_EO_sin_masa_Y.std
One model has the siemic masses in Y and the other doesn´t
Seismic masses are always lumped at the nodes of the member. The green beam has a node at the middle, so the mass at the node is considered and that impacts the bending moment diagram. The purple beam does not have a node at the middle, so the masses are at the ends of the beam only, which does not cause any BMD spikes at the middle. If you would insert a middle node on the purple beam, the results will be similar to the green beam.
Thanks Modestas. I understand. But if that is a local issue on that beam because of the existance (or not) of a middle node... what explains the important differences in the reactions values that i observe between the two cases?
You mentioned that the base shear differs if the masses in Y direction are removed. That is expected - different mass input will lead to different base shear. You should be providing the mass in all 3 global directions - X, Y and Z.